I. Field of the Invention
The present invention relates to an actuator assembly, particularly but not exclusively applicable to the support of motion platforms for vehicle simulators.
II. Related Art and Other Considerations
Vehicle simulators are widely used in training or entertainment applications. One major area of use is in the training of aircraft pilots.
In a typical aircraft flight simulator, a trainee pilot sits in a mock cockpit and views an image visible through the cockpit windows. Often the mock cockpit is supported on a motion platform so that the physical effects of vehicle motion can be simulated to supplement the simulation of aircraft motion represented by the visual image. Typically the mock cockpit is supported on six hydraulic actuators. The hydraulic actuators are connected between three pivots on the underside of the platform and three pivots on a support base beneath the platform. Thus each platform pivot is connected to two actuators which are in turn connected to respective ones of a pair of the support base pivots. In plan view, the pivots are arranged in a hexagonal shape, the platform pivots and support base pivots alternating around the periphery of the hexagon. This actuator system provides for movement about 6 axes providing translational (vertical, longitudinal or lateral) and rotational (pitch, roll and heave) displacements.
The conventional actuator system works well for standard sized flight simulators. If the size of the motion platform of the flight simulator is increased, the size of the conventional actuator system must be increased accordingly. This causes a problem when it is desired to use a large motion platform suitable for supporting for example a representation of a passenger aircraft fuselage to be used for emergency evacuation training, where the actuators would have to be very long.
The problem of large actuator size with large motion platforms can be addressed by widening the system base (ie providing six support base pivots so as to move the pairs of actuators further apart) and reducing the actuators maximum extensions. However, such an arrangement would cause problems of over-angling. That is to say the ends of the platform could strike the support base.
In some large motion platform systems, acceptable performances can be achieved with systems having three degrees of freedom. A three axis system has been proposed having three servo-controlled actuators and various methods of constraint, for example, "scissors", "hanging links" and cascade mechanisms where one axis is built upon another. Such three axis systems provide for pitch, heave and roll, but the methods of constraint and cascade mechanisms are complex and costly.